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Transport mechanisms in concrete and durability test methods

Dhanya B. S. Department of Civil Engineering, IIT Madras

National Workshop on Achieving Durable Concrete Construction Through Performance Testing

Common durability problems in concrete

Corrosion of steel in reinforced concrete

• Chloride induced corrosion

• Carbonation induced corrosion

Sulphate and other chemical attack

Alkali-aggregate reaction

Freezing and thawing

Why durability problems?

Typical section through an RC member

Cover-creteHeart-crete

Strength

Durability!!

Ballim, 2008

Why durability problems?

Concrete• Hydrated Cement Phase (HCP)• Aggregate phase• Interstitial Transition Zone (ITZ)

Gel pores, Capillary pores, Air voids Permeability and porosity

Aggressive species • Moisture, Sulphates, Chlorides, Carbon

dioxide, Oxygen, Alkalies etc.

TRANSPORT MECHANISMS IN CONCRETE

Diffusion Flow under concentration gradient

Gaseous diffusion• Through unsaturated concrete

Ionic diffusion• Through saturated and partially saturated concrete

Molecular diffusion • If the pores are relatively large

Equations of diffusion

C1

C2x

Equations of diffusion

C1

C2

x

+ Δt

Measuring diffusion coefficient

For steady state: measurement of chloride concentrations upstream and downstream

Unsteady state: Chloride ponding or immersion, measurement of chloride content along the depth of concrete and application of error function to determine diffusion coefficient

Permeation

Saturated liquid transfer controlled by a pressure gradient across the concrete

Ionic species dissolved in water also can move by permeation of water

Permeation higher when cracks and defects are present

Equations of Permeation

Water permeability

True indicator of concrete permeability to waterMeasures the steady state permeability

coefficient for water flow under a constant pressure head

Attainment of steady state conditions takes a long time

Can be modified to check the depth of water penetration by splitting the specimen

Measurement of flow rate

or

Measurement of Depth of penetrationaftersplitting

d

Measuring water permeability

Measuring Gas permeability

By measuring the pressure rise in a vacuum cell, placed on the concrete surface


By applying a pressure gradient across the test specimen and monitoring the pressure decay over


By measuring the gas flow though the concrete sample under a constant pressure gradient

Sorption/ Capillary water absorption

Uptake of liquid into unsaturated or partially saturated solids

Influenced by large capillaries and degree of continuity, compaction, aggregate orientation and distribution, mix composition etc.

Create conditions of capillary rise in concrete Measure the mass of water absorbed by the

concrete over time Test is done for 24 hours, although most of the

readings are taken within the first hour, as it is critical to get many data points in that period

Equation of Sorption

Measuring Sorption

Measure the mass of water absorbed by the concrete over time

Ma

√t

Curve through actual data points

Regression line through initial data pointsSlope = capillarity index, or sorptivity

Epoxy coatedsurface

Absorption

Refers to the bulk uptake of water unsaturated or partially saturated concrete

subjected to complete or partial immersion Easy to measureDifficult to penetrate all pores of concrete cannot get a true measure of porosity

Water absorption measurement

Simplest measurement• Measure oven dry mass, and saturated mass• Saturation generally done for at least 18 hours

Boiling water absorption test Vacuum saturation method

Migration/ Conduction

Movement of ionic species driven by difference in electrical potential

Nernst Planck Equation

Measurement of Migration

Electrical field is introduced into the test container by two electrodes which are connected to a potentiostat

Generally Cathode is the up-stream cell while the anode is the down-stream cell.

Wick action

Transport of liquid from a face in contact with liquid to drying face, where the liquid evaporates and dissolved ions precipitate as salts in the pores or on the surface of the medium

Wetting and drying cycles

DURABILITY TEST METHODS

Test methods to assess chloride penetration

Chloride Conductivity Test (SA DI Manual)

Rapid Chloride Permeability Test (ASTM C 1202)

Accelerated Chloride Migration Test (NORD 492)

Bulk diffusion Test (ASTM C 1556)

Test methods to assess gas penetrability

Oxygen Permeability Test (SA DI Manual)

Torrent Air Permeability Test (SIA 262/1)

Accelerated Carbonation Test

Test methods to assess water penetrability

Water Permeability (DIN 1048 part 5)

Water sorptivity Test (SA DI Manual)

Water Permeability(Germann)

Test method to assess concrete resistivity

Wenner four probe resistivity Test

Category Test method Parameter

Test methods to assess chloride penetration

Rapid Chloride Permeability Total charge passedAccelerated Chloride Migration Non-steady state diffusion coefficient

Chloride Conductivity Chloride ConductivityBulk diffusion Chloride concentration profile

Durability test methods and Parameters

Test methods to assess gas penetrability

Oxygen Permeability Oxygen Permeability IndexTorrent Air Permeability Coefficient of PermeabilityAccelerated Carbonation Carbonation depth

Test method to assess concrete resistivity

Wenner 4 Probe Resistivity Surface Resistivity

Test methods to assess water penetrability

Sorptivity Sorptivity indexGermann water Permeability Surface PermeabilityGerman water Permeability Water penetration depth

Transport mechanisms in concrete and test methods

Mechanism Definition Test methodSorption Capillary action SorptivityPermeation Flow under pressure Oxygen permeability, Torrent air

permeability (Gas permeability)Germann water Permeability, DIN 1048water penetration (Water permeability)

Diffusion Flow under concentrationgradient

Bulk diffusion, RCPT, Accelerated ChlorideMigration, Accelerated carbonation

Migration/Conduction

Movement due to appliedelectric field

RCPT, Accelerated Chloride Migration,Chloride conductivity, Wenner resistivity

Wick action Transport of ions or water froma face in contact with water todrying face

Sorptivity

Absorption bulk intake of water SorptivityAdsorption process of attachment of

molecules on the surfaceSorptivity

Convection Flow due to temperaturedifference

RCPT

Thank You…..

Wenner 4 Probe Resistivity Test

Method Merits Demerits

•Consists of four probes•Through two outer probes current is applied while the inner two probes measure the potential

• Quick method• Resistivity is used to calculate the corrosion rates in concretes•Avoids heating• Very low voltage (10V or lower) is applied for a short time

•Depends on pore solutionconductivity and moisture condition of the specimen•Inclusion of conductive materials affect the results

Resistivity, kΩ cm Concrete quality

> 100 Good50 - 100 Normal10 – 50 Poor< 10 Very poor

Classification Criteria (Andrade et al., 2004)

Rapid Chloride Permeability Test (ASTM C 1202)

Classification Criteria (ASTM C 1202)


•Specimen subjected to a 60V potential, for 6 hours• -ve side filled with 3% NaClsolution and +veside with 0.3M NaOH solution•Current is measured in each half hour, from which the total charge passed is calculated

•Widely used test method•Gives qualitative classification of concrete

•Current passed is related to all ions in the pore solution•Increase in temperature•Presence of conductive materials affects the test results•Many transport mechanisms act together

60V DC

#20 brass mesh

screen

0.3 N NaOH3% NaCl

Vacuum saturated specimens(100mm dia. , 50 mm thickness)

#20 brass mesh

screen

Accelerated Chloride Migration Test (NT Build 492)


•30 V potential is applied initially and after measuring the initial current, voltage is adjusted.•After specified duration the specimen is split and silver nitrate solution is sprayed•Depth of chloride penetration is used to calculate the non-steady state diffusion coefficient

•Capable of addressing some of the criticisms of RCPT related to the examination of actual chloride ion movement and temperature rise.

•Presence of conductive materials affects the test results•Many transport mechanisms acts together

Nordtest Method BUILD 492,Migration coefficient (m2/s)

Concrete quality

< 2 × 10-12 Very good2 – 8 × 10-12 Good8 – 16 × 10-12 Normal> 16 × 10-12 Poor

Classification Criteria (RILEM TC PSC –230 Draft)

Chloride Conductivity Test (DI Manual, SA)


•Consists of a two cell conduction rig in which concrete core specimens are exposed on either side to a 5M NaCl solution •Chloride conductivity is determined by measuringthe current flowing through the specimen due to the application of 10V potential difference

•Specimenspreconditioned before testing to standardize the pore water solution•Nullify the effect of other ions in the pore solution

•Destructive•Can not be applied on site

Chloride conductivity (mS/cm) Concrete quality

< 0.75 Very good0.75 – 1.50 Good1.50 – 2.50 Poor

> 2.50 Very poor

Classification Criteria:Alexander et al., 1999

Bulk diffusion test (ASTM C 1556)


•Natural diffusion under a very high concentration gradient •Specimens saturated with saturated lime water is immersed in 3% NaClsolution for 35 days•Uni-directional diffusion•Chloride profiling with profile grinder•Chloride ion concentration determined

•Identical to natural diffusion process•Other transport mechanisms are avoided

•Slow process•Powder can be lost during profiling•Effect of aggregates is unavoidable

Apparent diffusion coefficient (x 10-12 m2/s)

Resistance to chloride penetration

> 15 Low10 – 15 Moderate5 – 10 High2.5 – 5 Very High< 2.5 Extremely High

Classification criteria – Nilsson et al.1998

Oxygen Permeability Test (DI Manual, SA)


•Measures the pressure decay of oxygen passed through an oven dried, 30 mm thick slice of a 70mm diameter core placed in a falling head permeameter•The oxygen permeability index is defined as the negative log of the coefficient of permeability

•Good correlation with accelerated carbonation test

•Sensitive to macro- voids and cracks•Sensitive to the edges of the specimen

Oxygen permeability indexOPI

Concrete quality

> 10 Very good9.5 - 10 Good9.0 – 9.5 Poor

< 9 Very poor


Torrent Air Permeability Test Swiss Standard SIA 262/1-E:2003

Method Merits Demerits•Method is based on creating a vacuum on the surface of the concrete and monitoring the rate at which the pressure is raising in the test chamber after the vacuum pump has been disconnected

•Simple•Non destructive•Both lab and site application

•Sensitive to the moisturecondition of the specimen•Test is more sensitive to the surface condition of the specimen

Classification criteria:Torrent and Jacobs, 2009

Torrent kT×10-16m2 Concrete quality

< 0.01 Very good0.01 - 0.1 Good0.1 - 1.0 Normal1.0 - 10 Poor

> 10 Very poor

Accelerated Carbonation Test

Method Merits Demerits•Measuring the depth of carbonation at different sections of the prismatic samples at different time intervals (70,98, 112 and 154 days)•Phenolphthalein used as indicator – colourless implies carbonated

•Mechanism is diffusion, which is similar to the natural phenomena

•High degree of carbonation may lead to microstructuralalterations at high CO2concentrations

Sorptivity Test (DI Manual, SA)


•Measures the rate of movement of a water front through the concrete under capillary suction

•Simulates natural phenomena•Applicable to drilled cores

•Sensitive to macro-voids and cracks•Sensitive to the micro-structural properties of the near-surface zone of concrete•Destructive

Water sorptivitytest, mm/√h Concrete quality

< 6 Very good6 - 10 Good10 - 15 Poor

> 15 Very poor


Germann Water Permeability Test (GWT)


•A sealed pressure chamber is attached to the concrete surface.•Water is filled into the pressure chamber and a specified water pressure is applied to the surface•Pressure is kept constant using a micro-meter gauge with an attached pin that reaches into the chamber

•Non-destructive•Both field and lab test

•Pressure is not sufficient for impermeable concretes•Water flow may not be parallel to the gasket if pores are present

Coefficient of water permeability,

m2

Concrete quality

< 10-12 Good10-12 - 10-10 Normal

> 10-10 Poor

Classification Criteria (RILEM TC PSC –230 Draft)

Method Merits Demerits•Measure of the resistance of concrete against the penetration of water exerting pressure. •The test be done when the age of concrete is between 28 and 35 days•A water pressure of 0.5 N/mm2 is applied for a period of 3 days•After the pressure is released, the specimen is split into two and the depth of water penetration is noted.

•Depth of water penetration is measured

•Destructive•Air compressor is needed to keep the pressure constant

Water Permeability Test (DIN 1048 part 5)

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